The present invention relates to an RF A/B switch that has high isolation between the non-selected input and the output of the switch.
RF A/B switches are typically used to switch an input of an electronic receiver, such as a tape or disk player/recorder or a television, between two or more RF inputs. Such switches may be external of or internal to the receiver. External A/B switches usually comprise a housing having at least A and B inputs and an output that is coupled to an RF input of the receiver. The housing contains a switch, and a lever protruding through the housing is manually operated to move the switch so as to selectively couple one of the RF inputs to the output.
Internal A/B switches perform essentially the same function of switching a receiver between two or more RF inputs. Internal A/B switches are usually electronic in nature and often respond to a remote control in order to select one of a plurality of inputs. For example, in one known television receiver, the remote control may be used to switch the television receiver between two RF sources. These electronic A/B switches are typically complex in order to isolate the non-selected input from the output so that the signal on the non-selected input does not interfere with the signal from the selected input.
The present invention, at least in part, is directed to an electronic A/B switch that is less complex and/or expensive than prior art high performance electronic A/B switches and yet provides substantial isolation for the non-selected input up to UHF frequencies.
In accordance with one aspect of the present invention, an RF A/B switch associated with a receiver comprises a first input, a second input, an output, a first diode circuit, a second diode circuit, and a controller. The first diode circuit comprises a plurality of diodes and an impedance network coupled between the first input and the output. The second diode circuit comprises a plurality of diodes and an impedance network coupled between the second input and the output. The controller establishes a common series biasing current through at least one of the diodes in each of the first and second diode circuits. The common series biasing current biases one of the first and second diode circuits so as to configure a respective one of the impedance networks in a low pass filter configuration that couples a signal on one of the first and second inputs to the output, and the common series biasing current biases the other of the first and second diode circuits in a blocking configuration so as to block a signal on the other of the first and second inputs from the output.
In accordance with another feature of the present invention, the first diode circuit in its blocking configuration includes a first equivalent series impedance, the second diode circuit in its blocking configuration includes a second equivalent series impedance, and the RF A/B switch further comprises a first compensation impedance arranged to substantially cancel the first equivalent series impedance at a first predetermined frequency and a second compensation impedance arranged to substantially cancel the second equivalent series impedance at a second predetermined frequency.